Version/0.0.6

0. GIF format support (#33)
0. 'getPixel' method to get the color of a pixel (#44)
0. 'setPixel' method to set the color of a pixel (#45)
0. 'Contain' method (#68, #88)
0. 'Cover' method (#88)
0. Extend 'open' method to open raw pixels buffers (#86).
0. Run automated builds on Windows machine with AppVeyor (one step closer to distribution of pre-built binaries).

**Note**: giflib uses c99 features (`bool` type, specifically). This causes further restrictions when compiling on Windows, since c99 is only supported from VS2013. Until pre-built binaries are provided, users installing this module on Windows must have Visual Studio 2013 or later.